This invention relates to a program control apparatus and more particularly to a technique that can effectively be applied to a program controller such as a controller (processor) in a local-area network (LAN).
Generally, the interface between a transmission medium of a LAN and terminal equipment consists of a transceiver that performs conversion between electrical levels and logic levels and a controller that performs data buffering, assembly and disassembly of transmission formats such as packets, and network access control. Such LANs are described in the "Microcomputer Handbook," published by Ohm on Dec. 25, 1985, page 780.
The interface for the LAN may effectively be formed of a processor using a microprogram system. This is because when the interface is to support the data link layer in the open systems interconnection (OSI) reference model, complex protocol must be supported.
To develop processors of a microprogram system or debug LSI (large scale integration) circuits requires a trace function for tracing the flow of a program. Emulators and logic analyzers have trace memories to store the addresses of commands executed up to a breakpoint. Checking the contents of the trace memory allows one to know how the program ran before it reached the breakpoint. However, such emulators and logic analyzers are not only expensive but also require an LSI system to be designed to fit them.